1. Technical Field of Invention
This invention relates to beam formation and scanning antennas and more particularly to 2D and 3D wide angle steering antenna systems.
2. Background of Invention
A need exists for an antenna possessing 2D and 3D wide angle steering capabilities for circular polarization operation, while possessing a relatively low profile and being efficient and inexpensive. Examples of military and commercial applications where such an antenna can be employed abound. State of the art in microwave antennas, and more particularly, microwave antennas having wide angle scanning contains examples of antenna systems of similar capability, but characterized by a high degree of complexity, with expensive electronic components, which translates into costly fabrication.
At microwave frequencies, it is conventional to use slotted waveguide arrays, printed patch arrays, and reflector and lens systems. Conventional slotted planar array antennas have a complicated design, which, in conjunction with the precision and complexity required in the machining, joining, and assembly of such antennas, further limits their use. Printed patch array antennas suffer from inferior efficiency due to their high dissipative losses, particularly at higher frequencies and for larger arrays. Frequency bandwidths for such antennas are typically less than that which can be realized with slotted planar arrays. Sensitivity to dimensional and material tolerances is greater in this type of array due to the dielectric loading and resonant structures inherent in their design. On top of these we have to add the complexity of variable phase or time delay control to the elements, necessary in order to achieve full 3D scanning.
Reflector and lens antennas are generally employed in applications for which planar array antennas are undesirable, and for which the additional bulk and weight of a reflector or lens system is deemed to be acceptable. Reflectors are the least expensive antenna type for a large aperture. The absence of discrete aperture excitation control in traditional reflector and lens antennas limit their effectiveness in low sidelobe and shaped-beam applications. These systems can be scanned mechanically over limited angular regions with varying degrees of success.
This technology could also be applied to consumer electronics applications such as telecommunications (cellular telephone, back-haul, etc.) commercial aircraft, commercial radar, etc. where the distinct performance advantages and small form factor provided by the combination of RF MEMS and silicon germanium (SiGe) or other electronic circuits are desired.
Other traditional attempts involve the use of Reflectarray configurations with diode switches for steering. Reflectarrays are inexpensive and promising structures, and have been known for a long time [Berry, D. G., Malech, R. G. and Kennedy, W. A., xe2x80x9cThe reflectarray antenna,xe2x80x9d IEEE Trans., Ant. Propag., vol_AP-11, 1963, pp.646-651], however, when endowed with electronic scanning, in the form of complex arrays of independently controlled pin diodes, they become severely limited due to the complexities introduced by the presence of the DC bias network.
In U.S. Pat. No. 5,864,322, an electronic scan antenna for generating an electrically scanned RF beam in response to an incident RF beam includes a ground plane for reflecting the incident RF beam and a phasing arrangement of plasma structures operatively coupled to the ground plane. Each plasma structure includes gas containing areas which are reflective at the operating frequency range, when ionized. Each ionized plasma area, in cooperation with the ground plane, provides a portion of a composite RF beam which has a phase shift associated therewith. The antenna also includes a control circuit for selectively ionizing the gas containing areas such that the size of each ionized plasma area may be dynamically varied so as to dynamically vary the imparted phase shift. In this manner, the composite RF beam may be electronically scanned.
The latter scheme is essentially a reconfigurable dynamic FLAPS(trademark) antenna, a Reflectarray related technology. The FLAPS(trademark) antenna does not perform an electronic scanning function, and was disclosed in U.S. Pat. No. 4,905,014 to Gonzalez et al., entitled xe2x80x9cMicrowave Phasing Structures For Electromagnetically Emulating Reflective Surfaces And Focusing Elements Of Selected Geometry,xe2x80x9d issued on Feb. 27, 1990. This application is incorporated herein by reference.
More recently, in U.S. Pat. No. 5,905,472 [Wolfson, Ronald I., Milroy, William W., Lemons, Alan C. Coppedge, Stuart B., May 18, 1999, Assignee: Raytheon Company], a planar array antenna was presented that uses two distributed ferrite scanning line feeds to feed a planar array antenna. The scanning line feeds couple RF energy to the antenna from opposite sides to form a total of four beams offset in space that each cover different angular scan sectors. The scheme uses a 360 degree gimbal in the second axis, and is claimed to significantly improve on the performance of continuous transverse stub (CTS) antennas and systems. The scanning line feeds and planar array antenna may be designed so that the four scan sectors are contiguous, thereby increasing the angular scan coverage of the antenna at least fourfold. The switching matrix is used to sequentially feed each of four RF ports to effectively produce a single beam that scans over the four contiguous scan sectors. This application is incorporated herein by reference.
In accordance with one preferred embodiment of the present invention an antenna system includes a radiating element radiating signals through an array of phase-inducing resonant elements for focusing the radiated signals to produce a radiated beam which is scanned by rotating the array of phase of phase-inducing elements.
In accordance with another embodiment the characteristics of transmission through an FSS array of passive elements is combined with the steering behavior afforded by an induced phase gradient of an antenna aperture, for arbitrary polarized fields, and a mechanical fixture (implemented in the form of two rotating multilayered inhomogeneous panels, each layer of which possesses printed FSS element patterns resulting in each individual panel capable of 2-D scanning), to afford the type of aperture phase required to produce linear 3-D scanning. By cascading the two panels and rotating them with respect to each other, a field passing through the composite structure undergoes a phase change related to the vector addition of the two individual phase profile gradients of each plate. This results in 3D scanning. The scanning properties are complemented with the focusing requirements (which can be implemented in the form of an independent focusing multilayered FSS lens structure, or can be compounded with the scanning operation).
The resulting scheme can employ an arbitrary polarized feed illuminating the resulting planar and low profile structure, and resulting in no blockage. The antenna according to one embodiment of the present invention has the extra advantage of reduced complexity with much lower design and production costs than existing antenna systems. The resulting flat and simple 2D and 3D focusing and steering mechanism offers significant advantages over traditional electronic or electromechanical reflectarrays. It is based on well demonstrated concepts, has no feed blockage problems, no electronic switching, it is easy to build and is cheap.